This invention relates principally to a teletext decoder for teletext information comprising a plurality of different pages each of which is identified by a respective page number and each of which comprises a plurality of data packets each having a respective address; said teletext decoder including means for selecting a page by its page number, means for detecting a selected page number when received, and at least one page memory for storing received data packets of a selected page in a respective memory portion which is identified by the data packet address, the page memory being a write/read random access memory which data packets can be written into at the respective memory portions to over-write and thereby replace any data packets currently stored therein. The invention also relates to a method of processing teletext information.
The document "Broadcast Teletext Specification", September 1976, published jointly by British Broadcasting Corporation, Independent Broadcasting Authority and British Radio Equipment Manufacturers' Association, discloses a specification for transmitting teletext information in 625-line television systems.
Methods for enhancing the teletext specification given in the said "Broadcast Teletext Specification" are given in the document "World System Teletext and Data Broadcasting System-Technical Specification" December 1987, compiled by the Department of Trade and Industry of the U.K. Government.
The teletext information is usually transmitted as part of the television signal in television lines where no normal television picture information is present. Initially, teletext transmissions in the United Kingdom used as teletext data lines television lines 17 and 18 in first fields and television lines 330 and 331 in second fields, these lines being in the field or vertical blanking intervals (VBI), but current teletext transmissions in the United Kingdom now use at least six television lines in each VBI as data-lines. The VBI transmission is thus, in effect, multiplexed with the normal picture transmission and the teletext information transmitted in this way and representing data for display such as text and graphics can be displayed at a teletext television receiver as a selectable alternative to the normal picture information.
In the above-identified document "Broadcast Teletext Specification", a quantity of teletext information to be considered as an entity is termed a page and all of the pages which are available are normally transmitted in a recurrent cycle, with or without up-dating page information, as appropriate. At a teletext decoder any page can be selected, and the digitally coded data representing the page information is then acquired by the teletext decoder from the cyclic transmission and is stored in a page memory of the teletext decoder for as long as the page is required. A teletext decoder may have a multi-page memory having a plurality of memory portions in which individual pages can be stored.
A page memory of a teletext decoder is usually a write/read random access memory, and when new data is written into a storage location of the page memory it over-writes and thereby replaces any data previously stored in that location. As a consequence, if the data packets of an `old` page stored in the page memory are not erased before those of a `new` page are stored therein, the resulting total data in the page memory would include any data packets of the `old` page that have not been over-written by corresponding data packets of the `new` page. In other words, one or more spurious data packets may occur in every stored `new` page.
The said "Broadcast Teletext Specification" provides a page erasure interval by specifying, in effect, that data packets of a page will be transmitted such as to allow an active television field period between the transmission of a page-header of a page and the transmission of subsequent data packets for the page. This interval, which is approximately 20 ms. in 625-line television systems, is a page clearing interval which allows time for a teletext decoder to respond to receipt of the page-header of a new page selected by a viewer to erase from its page memory the data packets of any previously acquired page that is already stored therein, before the data packets of the new page are received. The page memory is thus made ready to store the data packets of the new page as transmitted in the data-lines of vertical blanking intervals subsequent to the vertical blanking interval in which the page-header was transmitted. This provision simplifies teletext decoder design by allowing the page clearing operation to be relatively slow.
With the majority of teletext decoders sold so far the method of clearing the data stored in a page memory was to use the page clearing interval to over-write any stored data by writing the character code for a `blank space` into every byte location of the page memory. However, this method imposes problems where a teletext decoder has several page memories which all have to be cleared in the page clearing interval, and/or where the page clearing interval is no longer available, as in the case of "full channel" operation in which television lines outside the vertical blanking interval are used for transmitting teletext information.
It is known from U.K. patent application GB No. 2,181,928A (PHB33219) to effect `pseudo` clearing of a page memory before the reception of a new page, by generating a "found" flag for each received data packet of a newly selected page and using these flags to control data acquisition and display. In particular, for data packets containing display information, `blank spaces` are displayed in the absence of their associated flags, and any corresponding spurious data packets left in the page memory from a previous one ignored. These `found` flags are respective bits which are stored in a `found` register which is reset in response to the detection of a selected page.
However, this prior means of `pseudo` clearing a page memory may have limitations due to the current trend in teletext information transmissions to include for various software control and extended display options many so-called extension data packets which are additional to the (24) data packets used for transmitting basic display information. In fact, latest teletext specifications permit up to 74 data packets to be associated with each teletext page for 625-line television systems. Therefore, if the bits representing `found` flags for all these data packets are packed into 74/8=10 8-bit byte locations it is necessary to write to all ten of these locations (which would form the `found` register) in the page clearing interval. In a worst case (e.g. in full channel operation), only the 64 .mu.s time for a television line is all that is available for acquisition of a data packet (40 bytes) including clearing of page memories, together with other possible control functions involved with the actual display. The actual time which could be allocated to clearing the page memories may be insufficient for the prior means of `psuedo`, clearing a page memory discussed above to be performed, in that when a given data packet is received by a teletext decoder it is necessary to calculate the byte and bit address of the `found` register where the associated `found` flag bit is stored and then to do a read/modify/write cycle to set only that bit and to leave the other bits in the same byte unchanged. This would require time-consuming logic.